Wide-band amplifiers are useful electronic circuits for a variety of applications. For example, wide-band amplifiers may be used in a high speed data communications system. In an optical fiber transmission system, a wide-band amplifier may be used in a data regeneration circuit for amplifying a high frequency electrical signal generated in response to a received optical signal.
Advantageously, such wide-band amplifiers should have a high frequency range, produce as little noise as possible, and have a tuneable or programmable gain. It is also desirable that the wide-band amplifier be amenable to a high level of integration. Furthermore, it is desirable for process and thermal variations to have as little influence on the operation of the wide-band amplifiers as possible.
Process variations arise due to the finite resolution of lithography in semiconductor fabrication. For example, while a pole 1/RC produced by the parasitic capacitance of a transistor can be predicted in theory, the manufacturing tolerance is 5-10%. This tolerance makes it difficult to compensate for such poles.
FIG. 1 shows a first prior art wide-band amplifier 100 called a differential cascode amplifier. See A. Grebene, Bipolar and MOS Analog Integrated Circuit Design, Chap. 8.5, p. 40-45 (1984). As shown, two cascode stages 110, 120 are connected in a differential amplifier configuration. The input voltage Vin is applied across the terminals 111, 112. By virtue of using cascode stages 110, 120, the Miller effect (build up of base-collector capacitance in the transistors 113, 114) is reduced thereby permitting higher frequency operation. However, the frequency response of the cascode differential amplifier 100 has an additional output pole which dominates as the output resistance driven by the amplifier increases. In addition, the amplifier 100 exhibits excess phase lag when used for gain-bandwidth optimization. Furthermore, the output swing of the amplifier 100 is reduced.
FIG. 2 shows another conventional wide-band amplifier 200 with neutralization capacitors 210, 220. See J. Mataya, G. Hanes & S. Marshall, "IF Amplifier Using C.sub.c Compensated Transistors," I.E.E.E. J. of Solid State Circuits, vol. SC-3, no. 4, p. 401-407, November, 1968. The capacitor 210 interconnects the base 211 of the transistor 215 with the collector 222 of the transistor 225. Likewise, the capacitor 220 interconnects the base 221 of the transistor 225 with the collector 212 of the transistor 215. The capacitance of the capacitor 210 is adjusted to be equal to the collector-base depletion capacitance of the transistor 215 and the capacitance of the capacitor 220 is adjusted to be equal to the collector-base depletion capacitance of the transistor 225. Thus, the effects of the collector-base depletion layer capacitances of the transistors 215, 225 are compensated by the antiphase transmissions of the capacitors 210, 220. Such a wide-band amplifier is disadvantageous because the capacitors 210 and 220 introduce a decreasing output pole in the frequency response of the amplifier 200. Thus, the amplifier 200 is not effective because it can't improve the bandwidth. That is, the dominant pole is now transferred to the output node and cannot be pushed away.
FIG. 3 shows a conventional wide-band amplifier 300 which utilizes a peaking technique. See M. Ohara, Y. Akazawa, N. Ishihara & S. Konaka, "Bipolar Monolithic Amplifiers for a Gigabit Optical Repeater," I.E.E.E. J. of Solid State Circuits, vol. SC-19, no. 4, p. 491-96, August, 1984. As shown, two transistors 310, 320 are connected in a differential configuration as before. The emitter of each transistor 310, 320 is connected via a capacitor 312 or 322 to a current mirror circuit 315 or 325, respectively. The bias current I.sub.D which flows through the transistors 314 or 324 (connected in a diode configuration) can be controlled by controlling the current mirror circuits 315, 325 thereby realizing a variable peaking function. The circuit produces an adjustable zero which depends on the capacitance and resistance of the transistors 314 or 324 (which capacitance and resistance depend on I.sub.D) and the capacitance of the capacitor 312 or 322. This zero is produced in front of the pole introduced by the emitter resistance 311 or 321, the capacitor 312 or 322, and the capacitance and resistance of transistor 314 or 324, respectively.
The problem with the circuit 300 is that the capacitances and resistances of the transistors 314 and 324 are also functions of temperature and are therefore subject to thermal drift. Furthermore, process variations produce an unpredictable response in the wide-band amplifier circuit 300. In addition, the wide-band amplifier circuit 300 is not suitable for implementation in CMOS.
By far the most commonly used class of wide-band amplifiers is feedback amplifiers. In general, a wide-band feedback amplifier sacrifices gain in order to increase bandwidth. R. Meyer & R. Blauschild, "A Wide-Band Low-Noise Monolithic Transimpedance Amplifier" I.E.E.E. J. of Solid State Circuits, vol. SC-21, no. 4, p. 530-33, August, 1986 discloses a shunt-series feedback amplifier using bipolar junction transistors.
FIG. 4 shows an illustrative conventional feedback amplifier 400 implemented in fine line NMOS. See K. Toh, R. Meyer, D. Soo, G. Chin & A. Voshchenkov, "Wide-Band Low-Noise, Matched Impedance Amplifiers in Submicrometer MOS Technology," I.E.E.E. J. of Solid State Circuits, vol. SC-22, no. 6, p. 1031-39, December, 1987. The amplifier circuit 400 is disadvantageous because in order to increase the bandwidth of the amplifier 400, a complicated multistage amplifier must be formed using an intricate S-parameter (scattering matrix parameter) analysis. For instance, the circuit 400 has transistors 430, 440 and 451-452 for amplification, transistor 460 forming an active shunt-shunt feedback circuit (from the transistor 451 to the transistor 430), and resistor 420 and capacitor 410 forming a passive feedback circuit (from the transistor 452 to the transistor 430). Furthermore, a zero is introduced in the frequency response of the amplifier 400 by the resistor 420 and capacitor 410.
FIG. 5 shows yet another conventional wide-band amplifier circuit 500 utilizing a parasitic capacitance compensation technique. See T. Wakimoto & Y. Akazawa, "A Low-Power Wide-Band Amplifier Using a New Parasitic Capacitance Compensation Technique," I.E.E.E. J. of Solid State Circuits, vol. 25, no. 1, p. 200-206, February, 1990. As shown, the outputs 511, 512 of a differential stage 510 are connected to an output parasitic capacitance compensation stage 520. The stage 520 includes transistors 521 and 522 which detect the voltages across the parasitic junction capacitances of the transistors 513 and 514, respectively, of the differential amplifier stage 510. The transistors produce currents which charge or discharge the capacitor 525 in a way such that the capacitance of the capacitor 525 is subtracted from the junction capacitance of the transistors 513 and 514 for purposes of calculating the gain and bandwidth of the amplifier circuit 500 (thereby increasing the bandwidth of the amplifier circuit 500). However, the parasitic capacitance compensation amplifier 500 has two disadvantageous features. First, the frequency response of the amplifier 500 experiences variations in the positions of the zero. Second, the step response of the amplifier 500 exhibits a relatively longer settling time.
It is therefore an object of the present invention to overcome the disadvantages of the prior art.